Researchers at WMG, University of Warwick have discovered a ‘window’ in which lithium-ion cells can be safely operated above the manufacturer limits.
The team at the UK institute tested cells using currents significantly higher than manufacturer limits and found that currents could be applied without thermal runaway.
The results were published in the paper, ‘Determining the Limits and Effects of High-Rate Cycling on Lithium Iron Phosphate Cylindrical Cells’ in the journal Batteries.
During testing, current fluxes were increased up to 100 C cycling conditions.
Charge and discharge current capabilities were possible at magnitudes of 1.38 and 4.4 times, respectively, more than that specified by the manufacturers’ claims. This increased current was applied for 500 charge-discharge cycles.
However, the application of these currents resulted in a rapid decrease in capacity in the first 60 cycles as well as an increase in resistance.
Furthermore, the application of such currents resulted in the increase of cell temperature, during both charge and discharge with natural convection during the rest step cooling the cell.
Formation of lithium plating from testing and ageing was also identified in the deformation of the “jelly roll” (coiled electrodes and separator).
Dr Melanie Loveridge, associate professor at WMG, told BEST: “Our message is that you can safely go beyond manufacturer’s stated limits but to the expense of capacity and lifespan, so it is not recommended.
“Small areas of plating won’t necessarily detonate catastrophic failure, but clearly will result in capacity fade. We think a safety aspect is relevant in the title as this chemistry does have good tolerance to high currents without instant failure or TR.
“Without parallel testing higher capacity cathodes, we could not assert comparative safety of other cells.
“Regarding the dendritic growth, LFP quite often has much smaller particle sizes compares with aggregate-type NMC etc, and so the lithium ions have shorter diffusion paths, and this certainly helps with plating / dendrite limitation.”
Justin Holloway (pictured), lead engineer at WMG, told BEST: “Safety of cells is very important, manufacturers place limits on allowable conditions (current, voltage and temperature for example) to ensure appropriate lifetime and safety.
We tested using currents significantly higher than manufacturer limits and found that currents could be applied without thermal runaway. We are therefore saying the tested cells are very safe so long as they are operated within the manufacturer limits.
“The typical features of the deformations is shown by the SEM image in the paper. We did not see any dendrite growth at the deformed areas. Visually we did see lithium plating in localised areas of the tested cells – however this did not appear to be consistently at the deformed areas.”
WMG’s Battery Forensic Group, led by Dr Loveridge, is keen to engage with industry and academia to grow advances in understanding new materials, battery performance and degradation modes.